1. Field of the Invention
The present invention relates to circuit boards and, more particularly, to bonding a non-corrosive surface metal, such as gold, to a contact finger of a circuit board.
2. Description of the Prior Art
In electronic apparatus, electronic components are frequently assembled in functional groups on printed wiring boards also referred to as circuit boards. These boards are commonly equipped with edge-type terminals or contact fingers. When the boards are inserted into respective circuit board connectors, the contact fingers become mechanically and electrically coupled to corresponding contacts in the connectors.
Most electronic logic circuits and many other functional circuits operate on low voltage and low current requirements. Mechanical connections in low voltage circuits tend to experience reliability problems if the conductive material used in respective contact surfaces are metals of the type that are likely to corrode. Corrosion on the contact surfaces tends to cause electrical discontinuities. When the voltages employed in corresponding circuits are insufficient to cause such discontinuities to be cleaned away by electrical arcing, the circuits fail. It is, therefore, common practice in the electronics industry to provide contact surfaces of conductors with non-corroding surface layers. Contact fingers on circuit boards are consequently frequently plated with gold.
Because of the material cost of gold, minimum usage is desirable. Consequently, only contact areas such as the contact finger of a circuit board, are commonly plated with a thin layer of gold after a circuit pattern on the board has been formed by conventional masking and etching techniques.
During such conventional processes and during the subsequent assembly of components to such a board, gold surfaces of the contact fingers at times become damaged. In circuit applications requiring high reliability circuits as, for instance, in communications systems, such damaged contact fingers are inacceptable because of a decreased reliability of the affected board. Repair of the damaged contact fingers by conventional plating methods is in general not possible because of the likelihood of causing further damage to the board and its circuit components during the chemical plating treatment.
Conventional gold plating methods of the contact fingers also require additional masking of the circuit portions not to be plated. The additional step is tedious and adds to the cost of the product. Consequently, in some commercial operations it may not even be feasible to invest in, and to maintain, gold plating facilities for the sole purpose of plating contact fingers.
As it becomes apparent from the above discussion, it is desirable to apply gold top surfaces to contact fingers by other methods than conventional gold plating. An alternative to such conventional methods is particularly desirable for the production of high reliability circuits, and for the repair of gold plated contact fingers that are found to be defective after the assembly of components to their respective boards.